Re: [PATCH] x86: mark BUG()s and assertion failures as terminal.

[Keir Fraser <keir.xen@gmail.com>]

On 19/09/2013 15:39, "Tim Deegan" <tim@xen.org> wrote:

> This helps avoid static analysis false-positives, and might lead to
> better code density as the compiler knows it doesn't have to restore
> spilled state &c.
> 
> Signed-off-by: Tim Deegan <tim@xen.org>
> ---
>  xen/include/asm-x86/bug.h  |   11 ++++++++---
>  xen/include/xen/compiler.h |    6 ++++++
>  2 files changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/include/asm-x86/bug.h b/xen/include/asm-x86/bug.h
> index e5dd559..956bfd2 100644
> --- a/xen/include/asm-x86/bug.h
> +++ b/xen/include/asm-x86/bug.h
> @@ -46,12 +46,17 @@ struct bug_frame {
>  
>  
>  #define WARN() BUG_FRAME(BUGFRAME_warn, __LINE__, __FILE__, 0, NULL)
> -#define BUG()  BUG_FRAME(BUGFRAME_bug,  __LINE__, __FILE__, 0, NULL)
> +#define BUG() do {                                              \
> +    BUG_FRAME(BUGFRAME_bug,  __LINE__, __FILE__, 0, NULL);      \
> +    unreachable();                                              \
> +} while (0)
>  
>  #define run_in_exception_handler(fn) BUG_FRAME(BUGFRAME_run_fn, 0, fn, 0,
> NULL)
>  
> -#define assert_failed(msg)                                      \
> -    BUG_FRAME(BUGFRAME_assert, __LINE__, __FILE__, 1, msg)
> +#define assert_failed(msg) do {                                 \
> +    BUG_FRAME(BUGFRAME_assert, __LINE__, __FILE__, 1, msg);     \
> +    unreachable();                                              \
> +} while (0)
>  
>  extern const struct bug_frame __start_bug_frames[],
>                                __stop_bug_frames_0[],
> diff --git a/xen/include/xen/compiler.h b/xen/include/xen/compiler.h
> index 7009a09..7d6805c 100644
> --- a/xen/include/xen/compiler.h
> +++ b/xen/include/xen/compiler.h
> @@ -14,6 +14,12 @@
>  #define always_inline __inline__ __attribute__ ((always_inline))
>  #define noinline      __attribute__((noinline))
>  
> +#if (!defined(__clang__) && (__GNUC__ == 4) && (__GNUC_MINOR__ < 5))

Do you mean for gcc-3.4 to use __builtin_unreachable()? This might be
clearer, correcter, and better match the prevailing compiler.h style, if it
was switched round to handle the __builtin_unreachable() case first.

 -- Keir

> +#define unreachable() do {} while (1)
> +#else
> +#define unreachable() __builtin_unreachable()
> +#endif
> +
>  #ifdef __clang__
>  /* Clang can replace some vars with new automatic ones that go in .data;
>   * mark all explicit-segment vars 'used' to prevent that. */